1. Field of the Invention
The present invention relates to a process for forming a polarization inversion layer on a substrate of ferroelectric crystal.
2. Description of the Related Art
Such a process for forming a polarization inversion layer is used in a method of manufacturing a wavelength converting element, which comprises an elongated core of a ferroelectric substance (three-dimensional waveguide) and a clad of a low refractive index enclosing the core and which causes a basic wave to enter through the waveguide and generates a second harmonics using quasi-phase matching (QPM). As a wavelength converting element which utilizes the second harmonic generation (SHG) and this quasi-phase matching (hereinafter also called QPM-SHG element), the one shown in FIG. 1 is known, which has a plurality of polarization inversion portions 3 (periodic domain inversion structure) that causes the polarization of a three-dimensional waveguide 2 where the basic wave propagates in a substrate 1 of ferroelectric crystal to be periodically inverted along the extending direction of the waveguide. The second harmonics output has such a property that as this output propagates, it periodically reaches the peak and trough levels every coherence length. Quasi-phase matching is a matching method which utilizes this property to alternately invert the sign of a polarization wave that is generated every coherence length, and adds the outputs of the second harmonics to increase the output. As the ferroelectric crystal substrate for the QPM-SHG element, there is used lithium niobate (LiNbO.sub.3) crystal or lithium tantalate (LiTaO.sub.3) crystal, which has an inversion characteristic (particularly the inversion of +Z face) by which the inversion of 180 degrees of the polarization in the direction of the Z axis of the crystal or the inversion of the polarization domain is likely to occur due to an external factor, such as an impurity, distortion stress, heat, or an electric field, and which also has a high non-linear optical coefficient. As shown in FIG. 1, a Z cut substrate 1 of a LiNbO.sub.3 crystal (substrate having, as a major surface, a Z cut face 1Z containing the X--Y axes with the Z axis of the crystal as a normal line) is mainly used as a wavelength converting element. In this diagram, the broken line arrows indicate the direction polarization.
As a method of forming a periodic domain inversion structure along a three-dimensional waveguide, heat treatment at near the Curie point, electron beam irradiation, application of an electric field, etc. are proposed. For instance, the electron beam irradiation method, disclosed in Japanese Unexamined Patent Publication No. Hei 3-121428, applies a voltage through a fine pattern of electrodes to form polarization inversion portions, as shown in FIG. 2. According to this method, with a Z cut substrate 1 of single domain LiNbO.sub.3 in use, both major surfaces, the .+-.Z cut faces, are sandwiched by pairs of electrodes 6 of opposite polarities, each pair facing each other, and a voltage of several tens of KV/cm is applied to this substrate 1 through the electrodes 6 at a high temperature of over 1000.degree. C., thus forming a plurality of polarization inversion portions 3, as shown in FIG. 3. The reason for applying a voltage to the Z cut face of the substrate is that the inversion of the polarization domain easily occurs at the Z cut face.
In the heat treatment at near the Curie point, proton exchange is conducted on the major surface of a Z cut substrate of LiNbO.sub.3 crystal or LiTaO.sub.3 crystal by pyrophosphoric acid, and the resultant structure is subjected to heat treatment at near the Curie point. Proton exchange is performed on the Z cut face of the substrate because the X cut face iX containing the Z--Y axes with the X axis of the crystal as a normal line and the Y cut face 1Y containing the Z-X axes with the Y axis the crystal as a normal line in FIG. 1 are corroded by proton exchange and are thus considered as improper for forming a proton exchange layer. This was reported by Kazuhisa YAMAMOTO, Kiminori MIZUUCHI and Tetsuo TANIUCHI in "Low-Loss Channel Waveguides in MaO:LiNbO.sub.3 and LiTaO.sub.3 by Pyrophosphoric Acid Proton Exchange" Jpn. J. Appl. Phys. Vol. 31 (1992), pp. 1059-1064 Part 1, No. 4, April 1992.
With a substrate of LiNbO.sub.3 crystal or LiTaO.sub.3 crystal in use, as described, a three-dimensional waveguide and a periodic domain inversion structure are formed on the Z cut face perpendicular to the Z axis where polarization inversion is easy to occur.
while the electron beam irradiation method of forming a periodic domain inversion structure in which the Z cut faces of the substrate are sandwiched by pairs of electrodes is advantageous in that the refractive indices of the polarization inversion portions do not change, this method requires the application of a high voltage according to the thickness of the substrate. As there is a limit to making the substrate thinner, there is also a limit to forming a fine periodic domain inversion structure.
In the method of performing a heat treatment at near the Curie point to form a periodic domain inversion structure, proton exchange is conducted so that some measures should be taken against the corrosion of the substrate.